Diving without air tanks, Pillion, 19, decided to go down for more fish. He never resurfaced. A friend eventually would find his body at the bottom of the gulf.

The Sept. 24 death of this St. Petersburg Catholic High graduate — a fit and experienced diver — offered a glimpse into the little-known world of free diving, a growing phenomenon that teeters on the edge of tragedy.

Over the last five years, free diving has gained popularity among Florida fishing enthusiasts and thrill-seekers. Without air tanks or heavy equipment, a diver can stalk fish more easily.

But the breath-holding technique carries a potentially fatal risk. A free diver can suddenly experience shallow-water blackout, a condition that causes the diver to faint.

"You think it is a casual sport," said Dave Coover, Pillion's stepfather, "but it isn't.''

Accurate free diving statistics are not available, but one diving interest group counted 34 deaths and four injuries in 2006. More than half of those happened in the United States, and 60 percent of U.S. deaths occurred in Florida.

Some experts put those numbers much higher and say deaths have increased in recent years because of the sport's increasing popularity.

Many free divers don't like to talk about the risk. Some don't know it's there until it's too late.

"Just at the point divers become very good, they become dangerous," said Terry Maas, a veteran free diver from California. "There is no forgiveness."

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Free diving isn't new. For centuries, Japanese and Mediterranean men and women used breath-holding techniques while collecting fish, sponges, pearls and other underwater treasures.

These days, despite the availability of scuba equipment and high-tech fishing techniques, the allure of moving about in the sea without tanks and tubes persists, often helped along by a machismo desire to push the body to its limits.

Free diving has gained widespread popularity in the United States only in the last decade. Florida has been the leading free diving state, with the Gulf of Mexico a prime arena.

The risks are rarely discussed outside of the more hard-core and competitive circles, and the physiology is tricky.

At the start of the dive, when the diver holds his breath, that oxygen is used by the body's tissues. As the dive continues, the oxygen begins to run out.

Because the brain is particularly sensitive to the lack of oxygen, the diver can instantly lose consciousness. Survivors of shallow-water blackout often do not recall signs of the blackout, such as dizziness or the need for air.

The blackouts seem to strike all ages and skill levels. Even elite free divers have died after running out of breath or passing out during dives.

Many amateur free divers — mostly young men — also have died in Florida. But the tragedies have been isolated enough that they have not drawn public alarm.

Some diving organizations, such as the Diver Alert Network, say up to 40 people die worldwide each year from free diving accidents. Even those aware of the danger can be victims.

Josh Choi, a student at Calvary Christian High School in St. Petersburg, was a well-trained diver who had been certified in scuba since he was 12. At one point, he could hold his breath for up to three minutes.

In March 2008, he went on a spearfishing trip in the Keys with his father, Sang, his three siblings and five friends. One of Josh's brothers saw Josh come up for air during a free dive, then disappear. He never came back up.

Over the years, Krack has seen dozens of accident reports that point to a disturbing pattern. People are free diving with little or no training. They're doing it alone or without a proper buddy system.

Krack said 90 percent of all shallow-water blackouts occur after the diver has resurfaced. After the blackout, divers get pulled down by their weight belts and begin to drown. And it's almost inevitable for those who dive regularly.

"If they are in the sport long enough," Krack said, "it will happen to them."

Maas, an oral surgeon with a background in marine biology, is somewhat of a legend in free diving. He is a former national champion who has written several books on the sport.

He calls shallow-water blackout a "widespread problem." So much so, he has invented a buoyancy vest designed to save divers. But it has been tough to develop and finance.

Still, Maas can talk with authority about the dangers of the sport. He warns others of how unforgiving the seas can be, how easy it is to push the limits, how many people make mistakes.

"Anybody can go out and buy a mask and fins and start free diving," he said. "Young people get into the sport and develop the skills very quickly. Unfortunately, (shallow water blackout) gives no warning."

He's an expert on the topic, though there's one cautionary tale he doesn't like to talk about.

Both brothers' skin turned blue-black, a symptom of severe hypoxia. Friends performed CPR, and they were rushed to a hospital. They eventually recovered, but their mother remembers the state of shock she was in when she arrived at the hospital that day in April 2008.

"I was stunned," Julie Richardson said. "I almost lost both my sons."

Since then, Richardson has become a strong advocate for free diving safety and runs a Web site, www.divewise.org.

"I am trying to start a dialogue with divers," she said.

It's not easy.

"There is a lot of macho associated with free diving," she said. "People don't want to talk about shallow-water blackout."

Emily Nipps can be reached at nipps@sptimes.com or (727) 893-8452.

Safety tips

Use a buddy system: Never dive alone or without someone constantly monitoring the dive.

Get proper training: Some certified dive instructors in the Tampa Bay area specialize in free diving and can explain weight belt safety and surface intervals.

Know your limits: Don't try to prove how long you can hold your breath underwater on a dive. Understand that the brain can shut down before the lungs.

What is shallow-water blackout?

Shallow-water blackout can occur without warning. In the latter part of a dive, oxygen levels drop in the body, with the brain being the most sensitive. A diver can lose consciousness in an instant and drown.